1. Field of the Invention
This invention relates to a liquid crystal display and a manufacturing method thereof, and more particularly to a liquid crystal display and a manufacturing method thereof capable of improving the visual angle.
2. Description of the Related Art
In a liquid crystal display or a liquid crystal cell, the orientation of liquid crystal molecules is changed from one particular state to other different state by some external action. The change in the optical property caused by the change of orientation off the liquid crystal molecules is utilized as a visual change in a display. Usually, the external action is application of an electric field. For orientating liquid crystal molecules in a particular state in the absence of an electric field, the surfaces of the glass substrates which sandwich a liquid crystal layer are usually subjected to orientation treatment.
In a conventional liquid crystal cell of twisted nematic (TN) type, the orientation treatment is achieved by rubbing the glass substrates, which sandwich a liquid crystal layer, with a cloth or the like in one direction, i.e. a so-called rubbing treatment.
A pair of substrates are assembled in such a way that the rubbing directions off the upper and lower substrates are orthogonal to each other. A liquid crystal cell filled with liquid crystal material is sandwiched between a pair of linear polarizers. When the liquid crystal cell is of negative display type, parallel polarizers are disposed, interposing the liquid crystal cell, in such a way that one of the rubbing directions is parallel to the polarization axis of the neighboring polarizer. In the case of positive display, crossed polarizers are disposed, interposing the liquid crystal cell, in such a way that the polarization axes are parallel to the rubbing direction of the neighboring substrate.
When orientation treatment is done by rubbing as stated above, orientated direction of the liquid crystal molecules become uniform on the substrate surface. Then, there occurs angular dependency of the display so that when the display is viewed by an observer, there exists a particular angular range in which the display can not be easily observed.
FIG. 4A shows an example of equi-contrast curves representing the visual angle characteristics of a TN type liquid crystal cell. In FIG. 4A, the normal direction to the liquid crystal cell is taken as .theta.=.0., and the polar angle .theta. from the normal, is shown radially with .theta.=.0. at the center. The direction in the horizontal plane (substrate surface) is represented by the azimuthal angle .phi.. These definitions are shown in FIG. 4B.
In FIG. 4A, each thick solid curve represents an equi-contrast curve. A contrast ratio (CR) value is attached to each curve. It can be seen in FIG. 4A, that the visual angle region with high contrast ratio is restricted in particular angular ranges. Therefore, such a liquid crystal cell has visual angle dependency that the cell is easily seen from particular directions but is not so easily seen from other directions.
When a liquid crystal cell having such visual angle dependency is utilized in a display, the contrast is extremely reduced at some angle with respect to the display surface (in case of FIG. 4A around .phi.=180 degrees). In a more extreme case, the black and white of display can be reversed.
The visual angle dependency as shown in FIG. 4A can be ascribed to the pre-tilt of liquid crystal molecules due to rubbing. The direction of pre-tilt of liquid crystal molecules coincides with the vector direction of rubbing.
When a voltage is applied to a liquid crystal cell, liquid crystal molecules begin to stand up along the pre-tilt direction of liquid crystal molecules. When the display is viewed from that direction, the polarization rotation is easily dissolved. Therefore, the terminating direction of the vector is the direction of easy observation.
Further, rubbing the substrate may generate electro-static charge by friction, which may cause dielectric breakdown of the orientation film, and hence a display defect by insufficient orientation of liquid crystal molecules thereat. Further, in a liquid crystal cell employing active matrix (AM) drive, rubbing a substrate, which has thin film transistors (TFT) or other drive elements and wirings formed thereon, may cause breakage of elements or wirings by the electrostatic charge caused by rubbing. In the case of a simple matrix display, the electrostatic charge caused by rubbing may break thin wirings.
Yet further, when an orientation film is formed on a substrate and rubbing treatment is done thereon, a great amount of fine particles is generated. Those particles may stick to the substrate by electrostatic force and may become a cause off inaccurate gap of the liquid crystal cell and/or of display defects such as black points and white points.